Yes, some apes do menstruate, a biological process involving the periodic shedding of the uterine lining. This involves the periodic shedding of the uterine lining in sexually mature females when pregnancy does not occur. It is a natural part of their reproductive cycle, preparing the uterus for a potential embryo each time.
Apes That Menstruate
All great apes, including chimpanzees, bonobos, gorillas, and orangutans, experience menstruation, much like humans. This highlights their close evolutionary relationship to humans.
Many other mammals, and even some primate groups like New World monkeys and prosimians, undergo estrous cycles. In an estrous cycle, the uterine lining is reabsorbed if pregnancy does not occur, rather than being shed. Some Old World monkeys, such as rhesus macaques and baboons, also exhibit overt menstruation, though the visibility of bleeding can vary significantly between species. In non-human apes, external signs of menstruation can be less obvious compared to humans, often due to behavioral differences like reabsorption of blood or less copious bleeding.
Understanding the Ape Menstrual Cycle
The menstrual cycle in apes is a cyclical process governed by fluctuating hormone levels, primarily estrogen and progesterone, mirroring the human cycle. These hormones orchestrate changes in the uterine lining, preparing it for a fertilized egg. If conception does not occur, the hormone levels decline, triggering the breakdown and shedding of the uterine lining.
While the underlying biology is similar, the visible signs of menstruation in apes are often subtle. For instance, gorillas experience minimal bleeding, which is frequently reabsorbed and not easily noticeable. Chimpanzee menstrual cycles vary, typically lasting around 37 days, though they can range from 28 to 45 days. Orangutans generally have cycles between 29 and 32 days, with menstruation lasting three to four days. Bonobos exhibit cycles of about 34 to 35 days, though some studies indicate a range of 31 to 51 days, and they can have a prolonged period of perineal swelling.
Evolutionary Significance of Menstruation in Apes
Menstruation in apes and humans represents a shared evolutionary trait, indicating a common ancestor. One theory suggests that menstruation evolved as a mechanism to prepare a fresh, receptive uterine lining for each potential pregnancy. This process ensures that the uterus is optimally prepared to accept a healthy embryo, rather than maintaining an old lining that might be less suitable.
Another hypothesis suggests menstruation serves as a protective mechanism against pathogens. By shedding the uterine lining, the body might remove potential infections or unhealthy cells, safeguarding reproductive health. The presence of menstruation in great apes, particularly with spontaneous ovulation and complex hormonal regulation, suggests an adaptation that allows for greater selectivity over embryo implantation, potentially rejecting embryos with significant genetic defects early on. This process is believed to offer long-term reproductive advantages in these primate lineages.